1. Field of the Invention
The present invention relates to a communication system, a signal receiving device, a signal transmitting device, and a communication method, in particular, to a communication system in which data hierarchized at the signal transmitting end are received at the signal receiving end after data layers to be received are selected according to receiving ability at the signal receiving end, a signal receiving device, a signal transmitting device in the communication system, and a communication method thereof.
2. Description of the Related Art
Digital modulations are employed in recent radio communications such as cellular phone communication systems or BS (Broadcasting Satellite) television broadcasting systems. Among the digital modulation schemes, QPSK (Quadrature Phase Shift Keying) and 16 QAM (sixteen-position Quadrature Amplitude Modulation) are typical ones used in digital mobile communications.
In the QPSK modulation scheme, the phase of the carrier is varied to represent more digital data states, and each symbol consists of two bits representing four phase states. FIG. 5 is a signal space diagram (also called phase diagram) showing the amplitude and the phase of the carrier in QPSK, where the I (In-Phase) axis represents the in-phase component, and the Q (Quadrature) axis represents the quadrature component, namely, the component 90 degrees phase-shifted from the reference axis I.
As shown in the signal phase diagram in FIG. 5, phases of the QPSK modulated carrier are located at the vertices of a square. QPSK allows the transmission of two bits of digital data in four phase states (00, 10, 11, and 01) with the phase positions of the carrier at 45, 135, 225 and 315 degrees, respectively.
A derivative of the above QPSK is π/4 shift-QPSK, which is used in a PDC (Personal Digital Cellular) cellular phone communication system, known as the second generation mobile communication system. In π/4 shift-QPSK, the phase positions of the carrier are shifted by π/4 relative to the corresponding phase positions in QPSK. π/4 shift-QPSK also allows the transmission of two bits of digital data in four phase states.
In addition, IMT-2000 (International Mobile Telecommunication-2000), known as the third generation mobile communication system, uses W-CDMA as the radio access scheme, and in the FDD mode of the W-CDMA scheme, BPSK (Binary Phase Shift Keying) is used in uplink data modulation, and QPSK is used in downlink data modulation. On the other hand, in the TDD mode of the W-CDMA scheme, QPSK is used in both uplink and downlink data modulations.
16 QAM varies both the amplitude and the phase of the carrier wave to represent more digital symbol points and thus more bits of digital data, allowing the transmission of four bits of digital data in sixteen phase states in each symbol.
FIG. 6 is a signal space diagram showing the amplitude and the phase of the carrier in 16 QAM, where numerals 0.9487, 0.3162, −0.3162, and −0.9487 indicate scales of I axis and Q axis.
In 16 QAM, the four bits in each symbol are divided into two groups each consisting of two bits. Combining the variation of the values of the two bit groups with the variation of the amplitude, sixteen symbol points are obtained, as illustrated in FIG. 6. Compared with the above QPSK at the same bit rate, the bandwidth occupied by 16 QAM is narrower than that of QPSK, and 16 QAM is more efficient in high speed digital data transmission using a narrow bandwidth, but is weak against fading (change of strength of the electrical wave due to change of time or the distance between the transmitter and the receiver). The above 16 QAM is used in digital MCA (for public radio service) and others.
As described above, QPSK and 16 QAM are typical digital modulation schemes used in digital mobile communications, and are used for transmission of hierarchized data together with another digital modulation scheme 64QAM.
In order to hierarchize data, for example, additional data for identifying data layers may be utilized. In this method, data and the additional data representing data layer classification are transmitted from a transmitter, and at a receiving end, the received signal is classified, and a user may use or extract desired information included in the received signal by making simple selection by using the additional data.
For example, Japanese Laid Open Patent Application No. 11-66085 discloses such a communication method.
In the above technique of the related art, the receiving ability of the receiving end is not considered. For this reason, when the receiving ability of the receiving end degrades relative to transmitting ability of the transmitting end, the utilization of the processing unit of the receiver (for example, CPU) rises, and data delay increases; consequently, communication quality is degraded.
In addition, in the case in which the transmitting device is a base station and the receiving device is a mobile station, the mobile station is weak against fading, and this is the environmental problem of a mobile communication system. The above prior art technique doe not take the influence of fading into account. For this reason, in the related art, communications are apt to be interrupted due to fading, and the communication quality is degraded.